The goal of this project is to understand the structure and assembly of calcium release units in adult and developing cardiac muscle. The units are formed at junctions between sarcoplasmic reticulum (SR) and surface membrane (and its invaginations, the transverse, T, tubules). At these junctions, the electrical signal of the surface membrane is transduced into a release of calcium from the SR. Two membrane molecules: feet (ryanodine receptors, SR calcium release channels) and slow calcium channels (dihydropyridine receptors, DHPRs) are involved int he release and in the events initiating it respectively. A third identified molecule is present at the junctions, the calcium binding protein calsequestrin. This project will explore the relationship between these three molecules in adult and developing cardiac muscle, using immunohistochemistry and two approaches to electron microscopy: freeze- fracture and selective staining. In adult muscle the key question is whether a specific spatial relationship, of the type present in skeletal muscle, exists between dihydropyridine receptors and feet. We hope to achieve a better understanding of the structures underlying the initiation of cardiac muscle activity, and to determine which structures are responsible for the significant differences in e-c coupling properties of skeletal and cardiac muscle. In developing muscle, the relationship between differentiation of transverse tubules, and the assembly of the three major proteins into functional units will be determined. Most of the initial work on development will be performed on chicken heart, where the developmental program, from initial formation of a functional contractile cell to the differentiation of a fully developed muscle fiber, is extended in time. Information from avian muscle will be correlated to rat muscle, whose physiology is best established.